This invention relates to electrophoretic displays in general and more particularly to a method and apparatus for increasing the life and response of such a display.
The prior art is replete with many references which teach and explain the operation of electrophoretic displays. Essentially, an electrophoretic display consists of a suspension of pigment particles dispersed in a dyed solvent of contrasting color. The solvent, as well as the particles, is injected into a cell which basically consists of two parallel and transparent conducting electrodes designated as the anode and cathode. Many such cells also employ a grid electrode which further controls the transportation of charged particles. In operation the charged particles are transported and forced against one electrode as the anode or cathode under the influence of an applied electric field so that the viewer may see the color of pigment which forms a desired pattern.
When the polarity of the field is reversed, the pigment particles are transported and packed on the opposite electrode. In any event, as indicated, the prior art is cognizant of such devices as well as undesirable effects in the operation of such devices. As the prior art understood, agglomeration and clustering are two natural phenomena which are associated with electrophoretic displays. As the resolution and speed of operation increases, these and other phenomena limit problems substantially effect the speed of operation as well as the life of the display. Agglomeration occurs when the particles in the suspension are forced into close proximity such as occurs when the pigment is compressed onto an electrode. Clustering occurs due to fluid motion within the cell and is accentuated as the fluid is switched back and forth since the particles migrate laterally which results in voids in the display.
Both phenomena have been considered by the prior art and have yet to be satisfactorily resolved by any of the prior art techniques. In order for a better understanding of these phenomena, reference is made to an article which appeared in the Journal of Applied Physics, September 1978 and entitled "The Understanding and Elimination of Some Suspension Instabilities in an Electrophoretic Display" by P. Murau and B. Singer pages 4820 to 4829. Other articles have been published which generally described the operating techniques and phenomena related with electrophoretic displays. See for example an article entitled "Electrophoretic Display Technology" by Andrew L. Dalisa, published in the IEEE Transactions on Electron Devices, July 1977. As one can understand from such prior art articles and other sources, the two primary sources of instability in such displays are agglomeration and clustering.
Pigment agglomerates, in suspension, occur when an insufficient barrier exists between pigment particles. Pigment agglomeration also occurs when pigment particles are packed tightly against an electrode such as occurs during the display mode of an electrophoretic cell. According to the prior art teachings, the cause of agglomeration in suspension can be eliminated with the use of certain copolymers. As far as clustering is concerned, this is caused by fluid disturbances in the vicinity of moving particles during transit in a cell. The size and pattern of these clusters are closely related to the amount of background charge in the suspension. The excess background charge consists of ionic charge carriers which differ in mobility. The slower moving charge carriers are found to cause turbulence which lead to pigment clusters. In any event, the prior art while cognizant of both phenomena did not formulate a successful solution to both problems. As the resolution increases, these phenomena reduce the effective life of the display and adversely affect the speed of operation.
In order to attempt to solve the phenomena of agglomeration, the prior art operated an electrophoretic display which was driven by a drive signal wherein the drive signal is modulated by an alternating voltage signal superimposed on the dirve signal and having a frequency sufficiently high to prevent observation.
This approach did not solve the clustering problems and further affected the quality of the implemented display. See U.S. Pat. No. 4,187,160 entitled Method and Apparatus for Operating an Electrophoretic Indicating Element, issued on Feb. 5, 1980 to A. Zimmermann. Furthermore, as the resolution and speed of operation of such displays increases then particle inertia affect the quality of life of the display.
It is therefore an object of the present invention to provide a method and apparatus for controlling the phenomena in an electrophoretic display.
It is a further object of this invention to control these phenomena associated with an electrophoretic display by utilizing an AC waveform of an appropriate frequency and wave shape and applying the waveform between the anode and the cathode electrodes until the pigment associated with the display is essentially suspended in the fluid medium and therefore not attached to any electrode. When this condition occurs, power is then removed and the pigment will remain in suspension until the panel is again activated by the necessary voltages to permit the display to operate accordingly.